1. Field of the Invention
The present invention relates to a distortion compensation apparatus and a distortion compensation method that compensate distortion of a power amplifier amplifying a transmission signal of a radio base station or the like by controlling an adjacent channel leakage power ratio to be small.
2. Description of the Related Art
In mobile communications systems to which W-CDMA (Wideband Code Division Multiple Access), PDC (Personal Digital Cellular) or the like is applied, it is necessary to control transmission power of a radio base station in a scope from approximately 10 mW to approximately 20 W, for example. As such a control means, transmission power control (TPC) such as an inner loop method, an open loop method, a closed loop method or the like is performed so as to control the power amplifier to obtain a desired transmission signal.
It is desirable to use the power amplifier that amplifies the transmitting signal in a linear region so as to make an amplification distortion small. However, in this case, power added efficiency is lowered to several percent and consumption of transmission power becomes large. The power added efficiency represents a ratio of a difference between the input power and output power to the input power. For example, in FIG. 1, an example of relationship among the output power [dBm], the power added efficiency [%] and the input power [dBm] is shown. A horizontal axis represents the input power [dBm], a left vertical axis represents the output power [dBm] and a right vertical axis represents the power added efficiency [%]. In FIG. 1, tendency of relationship between the output power and the power added efficiency is shown. That is, it is realized that the power added efficiency is very low when using only the linear region of the output power characteristics. Thus, a means has been employed in which the power added efficiency is improved by enabling the power amplifier to be used in a nonlinear region.
The amplification distortion becomes large when simply operating the power amplifier in the nonlinear region, causing leakage power to an adjacent channel to be large. Accordingly, a problem occurs in that an adjacent channel is interfered with. Thus, using a power amplifier having characteristics of wide linear region may be conceived. However, it is necessary to prepare the power amplifier having more capacity than required, thus an economical problem arises. Therefore, a structure using a linealizer (distortion compensation apparatus) compensating the distortion of the power amplifier has been in practical use.
For example, as shown in FIG. 2, when the distortion compensation is not performed, the transmission power characteristic is illustrated by a continuous curving line, and the leakage power to an adjacent channel between a one-dot chain line and a two-dot chain line becomes large. However, as shown by dotted lines, it is possible to reduce the leakage power to the adjacent channel by performing the distortion compensation.
In this case, an ACLR (Adjacent Channel Leakage Power Ratio) of the power which leaks to the adjacent channel to the transmitting power of the transmitting channel is equivalent to a ratio of an area of a spectrum between the one-dot chain lines representing the power of the transmission channel in FIG. 2 to an area of the spectrum between the one-dot chain line and the two-dot chain line representing the leakage power to the adjacent channel. This leakage power is strictly regulated so as to use frequency bands effectively since the leakage power becomes a noise component to the adjacent channel. Further, ACLR is the same as the ACPR (Adjacent Channel Power Ratio) used commonly.
Additionally, power of the channels adjacent to the transmission channel and power of the channels adjacent to the channels adjacent to the transmission signals are also regulated strictly. For example, in FIG. 3, P1 represents transmission power of a transmission band, PH1 represents leakage power to an adjacent channel having a higher frequency, PH2 represents leakage power to a channel having a further higher frequency, PL1 represents leakage power to an adjacent channel having a lower frequency, and PL2 represents the leakage power to a channel having a further lower frequency. A vertical axis represents power and a horizontal axis represents frequency. An adjacent channel leakage power ratio ACLR1 and the next adjacent channel leakage power ratio ACLR2 can be obtained by formulas as follows:
ACLR1=PH1 (or PL1)/P1
ACLR2=PH2 (or PL2)/P1
In this case, with regard to ACLR1, it is possible to take an average of PH1 and PL1 for a numerator of P1. Similarly, with regard to ACLR2, it is possible to take an average of PH2 and PL2 for a numerator of P1. In the following, ACLR1 and ACLR2 are referred to as ACLR other than a case where it is required to differentiate ACLR1 from ACLR2.
FIG. 4 is a schematic diagram showing a basic structure of the linealizer (distortion compensation apparatus) for performing the distortion compensation of the power amplifier. The linealizer includes a multiplier 110 which structures a pre-distortion part, an adaptive distortion compensation control part 111, a subtractor 112, and a power amplifier 113.
Additionally, f(p) represents a distortion function of the power amplifier 113. Illustrations of a directional coupler, a cymoscope and the like for branching a part of an amplified output signal of the power amplifier 113 are omitted.
The adaptive distortion compensation control part 111 receives a difference e(t) between the transmission signal x(t) and the amplified output signal. Then, the adaptive distortion compensation control part 111 inputs a distortion compensation signal to the multiplier 110. The distortion compensation signal thereof makes the difference e(t) become zero and corresponds to an amplitude or power of the transmission signal x(t). Thereby, a distortion in an opposite direction, that is, pre-distortion is given to the transmission signal x(t) so that the amplified output signal of the power amplifier 113 does not include a distortion component.
Additionally, the linealizer shown in FIG. 5 includes a multiplier 120, a distortion compensation signal memory 121, a distortion compensation signal generation part 122, a power amplifier 123 and a subtractor 124. The same as the basic structure shown in FIG. 4, the multiplier 120 gives the pre-distortion corresponding to a distortion function f(p) of the power amplifier 123 to the transmission signal x(t). Besides, the distortion compensation signal memory 121 stores a distortion compensation coefficient corresponding to a level or power of the transmission signal x(t). The distortion compensation signal generation part 122 receives the difference e(t) between the transmission signal x(t) and the amplified output signal so as to generate a distortion compensation signal, and updates the distortion compensation coefficient of the distortion compensation signal memory 121.
Furthermore, a structure of a linealizer (distortion compensation apparatus) shown in FIG. 6 is proposed in Japanese Laid-Open Patent Application No. 09-069733. In FIG. 6, the linealizer includes a multiplier 130, a distortion compensation table 131, a power calculation part (|x(t)|2) 132, a power amplifier 133, a subtractor 134, a complex number converter (conjg) 135, multipliers 136 through 138, an adder 139 and a directional coupler 140. In addition, f(p) represents a distortion function of the power amplifier 133, x(t) represents the transmission signal, e(t) represents a difference between the transmission signal and a signal which is branched from the amplified output signal by the directional coupler 140, xcexc represents a step size parameter and y(t) represents an output signal of the power amplifier 133.
Assuming that h(p) is a distortion compensation coefficient of the distortion compensation table 131, x, y, f, h, u, e are complex numbers, and * is a complex conjugate number, the multiplier 137 outputs u*(t) by multiplying hnxe2x88x921(p) with y*(t) which is obtained by branching an output signal y(t) of the power amplifier 133 by the directional coupler 140 and converting a value thereof into a complex conjugate number by the complex number converter 135. The multiplier 136 multiplies e(t) from the subtractor 134 and u*(t) from the multiplier 137. The multiplier 138 multiplies xcexc with e(t)xc2x7u*(t) from the multiplier 136. The adder 139 adds hnxe2x88x921(p) and xcexcxc2x7e(t)xc2x7u*(t). The distortion compensation coefficient hnxe2x88x921(p) is calculated by the following formulas and the distortion compensation table 131 is updated.
hn(p)=hnxe2x88x921(p)+xcexcxc2x7e(t)xc2x7u*(t)xe2x80x83xe2x80x83(1)
e(t)=x(t)xe2x88x92y(t)xe2x80x83xe2x80x83(2)
u(t)=x(t)xc2x7f(p)=h*nxe2x88x921(p)xc2x7y(t)xe2x80x83xe2x80x83(3)
hnxe2x88x921(p)xc2x7h*nxe2x88x921(p)=1xe2x80x83xe2x80x83(4)
y(t)=hnxe2x88x921(p)xc2x7x(t)xc2x7f(p)xe2x80x83xe2x80x83(5)
p=|x(t)|2xe2x80x83xe2x80x83(6)
Thus, the distortion compensation coefficient hn(p), which updates the distortion compensation table 131, is obtained by a formula as follows:
hn(p)=xcexcxc2x7y*(t)xc2x7h*nxe2x88x921(p)xc2x7e(t)+h*nxe2x88x921(p)
In this case, assuming y*(t)xc2x7h*nxe2x88x921(p)=u*(t), then the formula (1) is established. Additionally, a value of the formula (6) calculated by the power calculation part 132 becomes an address of the distortion compensation table 131, and the distortion compensation table 131 is updated by a result of the formula (1). Further, right side of the formula (4) is set to approximately 1 assuming that the amplification distortion of the power amplifier 133 is not large. Furthermore, by such a distortion compensation control, it is possible to reduce the leakage power to an adjacent band when operating the power amplifier 133 in the nonlinear region.
In the structure shown in the above-described FIG. 6, the step size parameter xcexc is set to be fixed in advance. When this step size parameter xcexc is set to be large, a convergence of the distortion compensation control has a tendency to be fast. However, there is a problem in that stability of the control is deteriorated. On the contrary, when the step size parameter xcexc is set to be small, there is a problem in that the convergence of the distortion compensation control becomes late. In this way, the step size parameter xcexc is determined by trade-off of the speed and stability of the convergence. That is, when the step size parameter xcexc is set. wrong, there is a problem in that the convergence time of the distortion compensation becomes long, or the distortion compensation control operates unstably.
Accordingly, it is a general object of the present invention to provide a novel and useful distortion compensation apparatus and distortion compensation method, in which the problems described above are eliminated.
Another object of the present invention is to adaptively control the step size parameter of the distortion compensation so as to realize the. convergence at high-speed and with stability after the convergence.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention, a distortion compensation apparatus that returns a part of amplified output signal of a power amplifier amplifying a transmission signal, reads a distortion compensation signal from a distortion compensation table, the distortion compensation signal corresponding to a signal calculated based on an error signal that is a difference between the transmission signal and the amplified output signal, a distortion compensation signal from the distortion compensation table and the step size parameter, and to a power of the transmission signal, multiplies the transmission signal with the distortion compensation signal thereof, and input the signal to the power amplifier, comprising a xcexc control part including a fast Fourier transformation part that obtains a spectrum of the amplified output signal, a calculation part that calculates an adjacent channel leakage power ratio and the like based on the spectrum, and a xcexc adjusting part that switches the step size parameter by comparing the calculated adjacent leakage power ratio with a threshold value.
Additionally, there is provided according to another aspect of the present invention, a distortion compensation apparatus that returns a part of an amplified output signal of a power amplifier amplifying signals of a plurality of carriers by a first combining part, reads a distortion compensation signal corresponding to a signal calculated based on an error signal of a difference between the returned signal and a transmission signal obtained by combining the transmission signals of the plurality of carriers by a second combining part, a distortion compensation signal from a distortion compensation table and a step size parameter, and to a power of the transmission signal combined by the second combining signal, multiplies the transmission signal with the distortion compensation signal thereof, adds the transmission signal combined by the first combining part to a compensation signal, the compensation signal being a difference between the multiplied output signal and the transmission signal combined by the second combining part, and inputs the signal thereof to the power amplifier, comprising a xcexc control part including a fast Fourier transformation part that obtains a spectrum of the amplified output signal, a calculation part that calculates an adjacent channel leakage power ratio based on the spectrum, and a xcexc adjusting part that switches the step size parameter by comparing the calculated adjacent channel leakage power ratio with a threshold value.
Additionally, there is provided according to another aspect of the present invention, a distortion compensation method that returns a part of an amplified output signal of a power amplifier amplifying a transmission signal, reads a distortion compensation signal from a distortion compensation table, the distortion compensation signal corresponding to a signal calculated based on an error signal that is a difference between the transmission signal and the amplified output signal, a distortion compensation signal from the distortion compensation table and the step size parameter, and to a power of the transmission signal, multiplies the transmission signal with the distortion compensation signal thereof, and input the signal to the power amplifier, comprising the steps of: obtaining a spectrum of the amplified output signal; calculating an adjacent channel leakage power ratio based on the spectrum; comparing the adjacent channel leakage power ratio with a threshold value; and switching the step size parameter to a small value when the adjacent channel leakage power ratio becomes smaller than the threshold value.
Additionally, there is provided according to another aspect of the present invention, a distortion compensation apparatus that returns a part of an amplified output signal of a power amplifier amplifying signals of a plurality of carriers by a first combining part, reads a distortion compensation signal corresponding to a signal calculated based on an error signal of a difference between the returned signal and a transmission signal obtained by combining the transmission signals of the plurality of carriers by a second combining part, a distortion compensation signal from a distortion compensation table and a step size parameter, and to a power of the transmission signal combined by the second combining signal, multiplies the transmission signal with the distortion compensation signal thereof, adds the transmission signal combined by the first combining part to a compensation signal, the compensation signal being a difference between the multiplied output signal and the transmission signal combined by the second combining part, inputs the signal thereof to the power amplifier, comprising the steps of: obtaining a spectrum of the amplified output signal; calculating an adjacent channel leakage power ratio based on the spectrum; comparing the calculated adjacent channel leakage power ratio with a threshold value; and switching the step size parameter.
According to the present invention, since the step size parameter (xcexc) in the distortion compensation control is switched by comparing the calculated adjacent channel leakage power ratio at the time t (ACLRt) with the threshold value, it is possible to realize convergence with higher speed at the beginning stage of the distortion compensation control by making a value of the step size parameter (xcexc) large, and to realize convergence stably after the convergence by making the value of the step size parameter (xcexc) small.
Further, by setting an initial value of the step size parameter (xcexc) corresponding to a desired transmission power value, it is possible to make the convergence of the distortion compensation control faster when the transmission power value is small. Additionally, since the step size parameter (xcexc) can be suitably varied according to a status of distortion compensation control by setting the threshold value for comparing with the adjacent channel leakage power ratio (ACLRt) to a value corresponding to the adjacent channel leakage power ratio (ACLRt) and further to the transmission power value, there is an advantage in that it is possible to control the distortion compensation toward the convergence state stably and with high speed.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the following drawings.